ResearchPad - catalysis https://www.researchpad.co Default RSS Feed en-us © 2020 Newgen KnowledgeWorks <![CDATA[Late‐Stage Diversification of Tryptophan‐Derived Biomolecules]]> https://www.researchpad.co/article/elastic_article_8325 Pd‐mediated reactions have emerged as a powerful tool for the site‐selective and bioorthogonal late‐stage diversification of amino acids, peptides and related compounds. Halotryptophans are accessible by biocatalytic approaches opening the application of a variety of cross‐coupling reactions for late‐stage modification of halotryptophan containing biomolecules.

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<![CDATA[Visible‐Light Controlled Divergent Catalysis Using a Bench‐Stable Cobalt(I) Hydride Complex]]> https://www.researchpad.co/article/elastic_article_7082 Night and day: A rare example of switching product outcome depending on light‐dark is reported. Using a cobalt(I) hydride complex, hydroborated products are obtained under visible‐light irradiation whereas, in contrast, alkene isomerisation occurs in the dark. This dual‐function catalysis results from alteration of the coordination sphere surrounding the metal centre.

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<![CDATA[Selective Acceptorless Dehydrogenation of Primary Amines to Imines by Core–Shell Cobalt Nanoparticles]]> https://www.researchpad.co/article/elastic_article_6948 The self‐dehydrogenation and hetero‐dehydrogenation of primary amines to their corresponding imines was achieved employing carbon supported cobalt nanoparticles coated with nitrogen‐doped graphene shell. The catalyst is obtained via pyrolysis of a polymer ionic liquid with a CoCl4 2− counter‐anion on a carbon support and shows unprecedented activity in these reactions.

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<![CDATA[Sterically Demanding Ag<sup>I</sup> and Cu<sup>I</sup> N‐Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity]]> https://www.researchpad.co/article/elastic_article_6909 A unique family of AgI and CuI complexes possessing sterically demanding N‐heterocyclic carbene ligands is described. Their structural and steric profiling reveals the highest buried volumes (%V bur up to 57.5) for any expanded‐ring backbone NHC complex. For the first time, the AgI complexes have been employed as ligand‐transfer reagents while the CuI complexes proved to be efficient candidates for catalysis (see figure).

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<![CDATA[A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light‐Mediated Cycloadditions]]> https://www.researchpad.co/article/elastic_article_6757 Bind me tender: Cyclic enone‐3‐carboxylic acids (X=CH2, O) display a binding motif (in blue) for an assembly with chiral acid 1. The acid acts as photocatalyst by transferring the energy of visible photons to the substrate via its thioxanthone substituents (in yellow) enabling an enantioselective [2+2] photocycloaddition (see scheme).

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<![CDATA[Synthesis of Fulvene Vinyl Ethers by Gold Catalysis]]> https://www.researchpad.co/article/elastic_article_6668 Regioselective switch: Gold‐catalyzed cyclization of 1,5‐diynes with ketones as reagents and solvent provides diversely substituted vinyl ethers under mild conditions. The regioselectivity of such gold‐catalyzed cyclizations is usually controlled by the scaffold of the diyne. In this work, the first solvent‐controlled switching of regioselectivity from a 6‐endo‐dig‐ to 5‐endo‐dig‐cyclization in these transformations, providing fulvene derivatives is reported (see scheme).

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<![CDATA[Synthesis of Highly Fluorinated Arene Complexes of [Rh(Chelating Phosphine)]+ Cations, and their use in Synthesis and Catalysis]]> https://www.researchpad.co/article/N5bf02705-1d78-491f-a090-ccdec03740a6

Abstract

The synthesis of rhodium complexes with weakly binding highly fluorinated benzene ligands is described: 1,2,3‐F3C6H3, 1,2,3,4‐F4C6H2 and 1,2,3,4,5‐F5C6H are shown to bind with cationic [Rh(Cy2P(CH2)xPCy2)]+ fragments (x=1, 2). Their structures and reactivity with alkenes, and use in catalysis for promoting the Tishchenko reaction of a simple aldehyde, are demonstrated. Key to the synthesis of these complexes is the highly concentrated reaction conditions and use of the [Al{OC(CF3)3}4] anion.

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<![CDATA[Acetylene as a Dicarbene Equivalent for Gold(I) Catalysis: Total Synthesis of Waitziacuminone in One Step]]> https://www.researchpad.co/article/N142732d0-ffd6-45f0-bbe2-9deb5701faa2

Abstract

The gold(I)‐catalyzed reaction of acetylene gas with alkenes leads to (Z,Z)‐1,4‐disubstituted 1,3‐butadienes and biscyclopropanes depending on the donor ligand on gold(I). Acetylene was generated in situ from calcium carbide and water in a user‐friendly procedure. Reaction of acetylene with 1,5‐dienes gives rise stereoselectively to tricyclo[5.1.0.02,4]octanes. This novel double cyclopropanation has been applied to the one step total synthesis of the natural product waitziacuminone from acetylene and geranyl acetone.

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<![CDATA[Stereochemistry of the Reaction Intermediates of Prolinol Ether Catalyzed Reactions Characterized by Vibrational Circular Dichroism Spectroscopy]]> https://www.researchpad.co/article/Nf7d7f78a-ea9a-4de1-9780-ad36a2645b72

Abstract

Spectroscopic characterizations of key reaction intermediates are often considered the final confirmation of a reaction mechanism. This proof‐of‐principle study showcases the application of vibrational circular dichroism (VCD) spectroscopy for the characterization of in situ generated reaction intermediates using the key intermediates of enamine catalysis of Jørgensen–Hayashi‐type prolinol ether catalysts as model system. By comparison of experimental and computed spectra, the enamines are shown to preferentially adopt an anti‐conformation with E‐configured C=C bond. For the parent prolinol catalyst, the structure and stereochemistry of the oxazolidine side product is determined as well. This study thus demonstrates that VCD spectra can provide insights into structural preferences of organocatalysts that utilize a covalent activation mechanism. Thereby it outlines new fields of applications for VCD spectroscopy and finally adds the technique to the toolbox of physical organic chemistry for in‐depth mechanistic studies.

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<![CDATA[Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO 2 (H 2 O) n − , n =1–10, in the C−O Stretch Region]]> https://www.researchpad.co/article/Nc926e130-88c2-4f8e-8aae-cb055fa680d9

Abstract

We investigate anionic [Co,CO2,nH2O] clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm−1 using an FT‐ICR mass spectrometer. We show that both CO2 and H2O are activated in a significant fraction of the [Co,CO2,H2O] clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 . However, calculations find Co(HCOO)(OH) as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm−1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH). Upon additional hydration, all species [Co,CO2,nH2O], n≥2, undergo IRMPD through loss of H2O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm−1, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO ligand around 1580–1730 cm−1, and the symmetric C−O stretching mode of the HCOO ligand around 1300 cm−1. A weak feature above 2000 cm−1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

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<![CDATA[Bis‐Cyclometalated Indazole Chiral‐at‐Rhodium Catalyst for Asymmetric Photoredox Cyanoalkylations]]> https://www.researchpad.co/article/Nc8ed3d4d-972e-43c1-ad80-85532cf6bc34

Abstract

A new class of bis‐cyclometalated rhodium(III) catalysts containing two inert cyclometalated 6‐tert‐butyl‐2‐phenyl‐2H‐indazole ligands and two labile acetonitriles is introduced. Single enantiomers (>99 % ee) were obtained through a chiral‐auxiliary‐mediated approach using a monofluorinated salicyloxazoline. The new chiral‐at‐metal complex is capable of catalyzing the visible‐light‐induced enantioselective α‐cyanoalkylation of 2‐acyl imidazoles in which it serves a dual function as the chiral Lewis acid catalyst for the asymmetric radical chemistry and at the same time as the photoredox catalyst for the visible‐light‐induced redox chemistry (up to 80 % yield, 4:1 d.r., and 95 % ee, 12 examples).

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<![CDATA[New Insights into the Catalytic Activity of Cobalt Orthophosphate Co3(PO4)2 from Charge Density Analysis]]> https://www.researchpad.co/article/Nbc1e5c01-be8e-440b-8f7b-a46aa8ea82f7

Abstract

An extensive characterization of Co3(PO4)2 was performed by topological analysis according to Bader‘s Quantum Theory of Atoms in Molecules from the experimentally and theoretically determined electron density. This study sheds light on the reactivity of cobalt orthophosphate as a solid‐state heterogeneous oxidative‐dehydration and ‐dehydrogenation catalyst. Various faces of the bulk catalyst were identified as possible reactive sites given their topological properties. The charge accumulations and depletions around the two independent five‐ and sixfold‐coordinated cobalt atoms, found in the topological analysis, are correlated to the orientation and population of the d‐orbitals. It is shown that the (011) face has the best structural features for catalysis. Fivefold‐coordinated ions in close proximity to advantageously oriented vacant coordination sites and electron depletions suit the oxygen lone pairs of the reactant, mainly for chemisorption. This is confirmed both from the multipole refinement as well as from density functional theory calculations. Nearby basic phosphate ions are readily available for C−H activation.

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<![CDATA[Merging Regiodivergent Catalysis with Atom-Economical Radical Arylation]]> https://www.researchpad.co/article/N3dc7120b-257c-462b-bb8c-c3321cbbe9d5

Abstract

A titanocene‐catalyzed regiodivergent radical arylation is described that allows access to either enantiomerically pure tetrahydroquinolines or indolines from a common starting material. The regioselectivity of epoxide opening that results in the high selectivity of heterocycle formation is controlled by two factors, the absolute configuration of the enantiopure ligands of the (C5H4R)2TiX2 catalyst and the inorganic ligand X (X=Cl, OTs). The overall reaction is atom‐economical and constitutes a radical Friedel–Crafts alkylation.

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<![CDATA[Study on dielectric properties of high organic sulfur coking coal and modeling sulfur compounds]]> https://www.researchpad.co/article/5c37b795d5eed0c48449054b

Coking coal is geologically scarce resource and most of them cannot be directly used in steel making due to their higher sulfur content. One desulfurization method that has great potential for massive application is microwave desulfurization, which removes the relatively stubborn organic sulfur under mild conditions. The dielectric properties of coals determine the efficiency of the microwave energy absorption. The key to describing the mechanism of microwave desulfurization and further improvement of the desulfurization efficiency is the dielectric response of organic sulfur compounds in coal to microwave. This study focuses on existing formand microwave response of organic sulfur components of three typical coking coal in China. Resultsshowed that the major organic sulfur in selected coals is thiophene which has a stable structure and is the most difficult to be removed. Several dielectric peaks (dielectric loss)andsignificant differencesofeach selected coal samples are observed. The microwave absorption peaks of the model sulfur compounds are identified to be within 9-11GHz. The real parts of the relative dielectric constants (hereinafter referred to as ε′) shows a decreasing trend as: diphenyl sulfoxide > diphenyl sulfone > diphenyl sulfide > dibenzothiophene > Octadecane thiol. Response to microwaveare observed to be distinctively different between sulfur-containing and sulfur-free model compounds. The dielectric polarization of mixture (coal mixed with model sulfur compounds) is greater than pure coal. Meanwhile the higher the sulfur content of the coal, the greater the ε′ is. Sulfur componentsin coal can significantly influence its polarization.

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<![CDATA[Structural insight into substrate and product binding in an archaeal mevalonate kinase]]> https://www.researchpad.co/article/5c12cf8ed5eed0c484914932

Mevalonate kinase (MK) is a key enzyme of the mevalonate pathway, which produces the biosynthetic precursors for steroids, including cholesterol, and isoprenoids, the largest class of natural products. Currently available crystal structures of MK from different organisms depict the enzyme in its unbound, substrate-bound, and inhibitor-bound forms; however, until now no structure has yet been determined of MK bound to its product, 5-phosphomevalonate. Here, we present crystal structures of mevalonate-bound and 5-phosphomevalonate-bound MK from Methanosarcina mazei (MmMK), a methanogenic archaeon. In contrast to the prior structure of a eukaryotic MK bound with mevalonate, we find a striking lack of direct interactions between this archaeal MK and its substrate. Further, these two MmMK structures join the prior structure of the apoenzyme to complete the first suite of structural snapshots that depict unbound, substrate-bound, and product-bound forms of the same MK. With this collection of structures, we now provide additional insight into the catalytic mechanism of this biologically essential enzyme.

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<![CDATA[Engineered Sulfur-Resistant Catalyst System with an Assisted Regeneration Strategy for Lean-Burn Methane Combustion]]> https://www.researchpad.co/article/5b59aca7463d7e792f619095

Abstract

Catalytic combustion of methane, the main component of natural gas, is a challenge under lean‐burn conditions and at low temperatures owing to sulfur poisoning of the Pd‐rich catalyst. This paper introduces a more sulfur‐resistant catalyst system that can be regenerated during operation. The developed catalyst system lowers the barrier that has restrained the use of liquefied natural gas as a fuel in energy production.

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<![CDATA[A modified expression of the major hydrolase activator in Hypocrea jecorina (Trichoderma reesei) changes enzymatic catalysis of biopolymer degradation]]> https://www.researchpad.co/article/5bcf6ad940307c74ebb862ac <![CDATA[Intramolecular N–H⋯Cl hydrogen bonds in the outer coordination sphere of a bipyridyl bisurea-based ligand stabilize a tetrahedral FeLCl2 complex]]> https://www.researchpad.co/article/5ba6c6fb40307c39ee105b36

A bipyridyl-bisurea ligand coordinates a tetrahedral FeCl2 complex and demonstrates secondary coordination sphere influence through intramolecular hydrogen bonding to the chloride ligands.

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<![CDATA[Kinetic Analysis of an Efficient DNA-Dependent TNA Polymerase]]> https://www.researchpad.co/article/5b03b68f463d7e6b556dacbf

ja-2004-028255_0001.jpgα-l-Threofuranosyl nucleoside triphosphates (tNTPs) are tetrafuranose nucleoside derivatives and potential progenitors of present-day β-d-2‘-deoxyribofuranosyl nucleoside triphosphates (dNTPs). Therminator DNA polymerase, a variant of the 9°N DNA polymerase, is an efficient DNA-directed threosyl nucleic acid (TNA) polymerase. Here we report a detailed kinetic comparison of Therminator-catalyzed TNA and DNA syntheses. We examined the rate of single-nucleotide incorporation for all four tNTPs and dNTPs from a DNA primer−template complex and carried out parallel experiments with a chimeric DNA−TNA primer−DNA template containing five TNA residues at the primer 3‘-terminus. Remarkably, no drop in the rate of TNA incorporation was observed in comparing the DNA−TNA primer to the all-DNA primer, suggesting that few primer-enzyme contacts are lost with a TNA primer. Moreover, comparison of the catalytic efficiency of TNA synthesis relative to DNA synthesis at the downstream positions reveals a difference of no greater than 5-fold in favor of the natural DNA substrate. This disparity becomes negligible when the TNA synthesis reaction mixture is supplemented with 1.25 mM MnCl2. These results indicate that Therminator DNA polymerase can recognize both a TNA primer and tNTP substrates and is an effective catalyst of TNA polymerization despite changes in the geometry of the reactants.

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<![CDATA[A Catenane Assembled through a Single Charge-Assisted Halogen Bond**]]> https://www.researchpad.co/article/5aeb9f27463d7e2dfdbce398 ]]>